1. Field of the Invention
The present invention relates to a process for oligomerizing isobutene an isobutenic hydrocarbon mixture over a solid acidic ion exchange resin, said acidic ion exchange resin containing sulfonic acid groups, some of whose protons have been exchanged for metal ions.
2. Discussion of the Background
A mixture of 2,4,4-trimethylpent-1-ene and 2,4,4-trimethylpent-2-ene, usually referred to in industry as diisobutene, is obtained on an industrial scale by dimerizing isobutene. Especially after hydrogenation to the corresponding paraffin (isooctane, 2,2,4-trimethylpentane), it is a sought-after fuel component owing to its high octane number (a measure of the knock resistance of the carburetor fuel). Isooctane also serves as a reference for determining the octane number. For use as a fuel additive, mixtures of diisobutene or isooctane can also be used which contain other C8 isomers or hydrocarbons having other carbon numbers.
For use in syntheses, relatively high purities of the diisobutene are normally desired. Hydroformylation of diisobutene provides 3,5,5-trimethylhexanal which can be oxidized to the corresponding carboxylic acid. The carboxylic acid finds use for preparing peroxides, lubricants and siccatives. Diisobutene is also used for alkylating phenols. The resulting alkylaromatics are intermediates for the production of detergents.
Diisobutene is obtainable by the dimerization, generally oligomerization, of isobutene. In addition to the dimers (C8), this also results in oligomers of higher molar masses (mainly C12, C16). As a result of framework rearrangement reactions, the C8-dimers also contain small proportions of other C8-olefins in addition to diisobutene. When the reactant used for the oligomerization is isobutene in mixtures with other olefins, cooligomers are additionally formed. In the presence of 1-butene, 2,2-dimethylhexenes and 2,2,3-trimethylpentenes, for example, are formed.
For the economic preparation of diisobutene, whether it be as a reactant for syntheses or as a fuel additive, from isobutenic hydrocarbon mixtures, several criteria are to be observed. These include a high C8 selectivity, a high C8 isomer purity, the long-term stability of the catalyst, the technical solutions for removing the heat of reaction generated and, in the case of mixtures which also contain 1-butene, a low isomerization of the 1-butene to 2-butenes.
Among other reasons, a high C8 selectivity is desired, since the tetramers of isobutene (or cotetramers with linear C4-olefins) or their hydrogenated derivatives are unsuitable for use in carburetor fuels, since they have too high a boiling point. The formation of tetramers in the preparation of fuel additives thus constitutes a real yield loss. The boiling points of the trimers and of the hydrogenated derivatives at 170–180° C. lie within the upper part of the boiling point range for carburetor fuels; although they can be used proportionately in fuels, their formation is nevertheless to be substantially minimized.
A high C8 isomer purity, i.e. a high proportion of diisobutene in the C8 fraction and only slight formation of codimers (dimer of 1-butene and isobutene) and rearrangement products, is desired especially in the preparation of diisobutene which finds use in syntheses. In addition, the formation of codimers with, for example, n-butenes is to be avoided, since this consumes n-butenes which could otherwise be used in other ways (1-butene, for example, as a (co)monomer for polymers).
High long-term stability of the catalysts is necessary not only to minimize the catalyst costs, but also to keep the technical cost and inconvenience of the catalyst change low.
The oligomerization of isobutene additionally releases heat in considerable amounts. When this is not removed to a sufficient extent, the reaction mixture heats up. This can lead both to deterioration in the selectivities and to an adverse effect on the catalyst stability.
Isobutene is often obtained industrially in mixtures with 1-butene. It is not possible to separate the materials by distillation at technically viable cost and inconvenience. A separation is therefore achieved by selective chemical reaction of one of the two components, for example etherification of the isobutene. However, the chemical conversion of the isobutene must not result in the rearrangement of the 1-butene to 2-butenes (cf. K. Weissermel, H. J. Arpe, Industrielle Organische Chemie, Wiley-VCH, 5th Edition, 1998, page 74–82). 1-Butene is a sought-after raw material, and, among other uses, it is used as a comonomer in the preparation of polyolefins.
All of these criteria have already been addressed in the literature and some solutions have also been found. One of the main emphases of the work is in the development of catalysts. The oligomerization of isobutene can be catalyzed by Lewis or Bröonsted acids, or coordination compounds, in particular those of nickel. Catalysts for this reaction have been known for some time and are the subject-matter of numerous patents and publications.
The oligomerization can in principle be carried out homogeneously, i.e. using catalysts soluble in the reaction mixture, or heterogeneously, i.e. using catalysts insoluble in the reaction mixture. The disadvantage of the homogeneous process is that the catalyst leaves the reactor with the reaction products and unconverted reactants, from which it has to be removed, worked up and disposed of or recycled.
Most of the industrial processes therefore use heterogeneous catalysts which are additionally often arranged in a fixed bed, so that there is no need for a costly or inconvenient catalyst removal. Most of the existing fixed bed catalysts belong to one of the following groups:
a) mineral acids (e.g. sulfuric acid or phosphoric acid) on a support material (e.g. alumina or silica)
b) the zeolites or other aluminum silicates, undoped or doped by further metals, in particular with transition metals
c) acidic ion exchange resins.
Mineral acids on supports have little suitability as catalysts, since they also promote framework rearrangements (reaction of two molecules of isobutene to give C8-isomers other than 2,4,4-trimethylpentene).
In EP 0 224 220, a butene oligomerization is carried out over a zeolite catalyst doped with bismuth and/or lead. The C8 fraction contains over 4% of undesired 2,3,4-trimethylpentenes. Zeolites are likewise used as catalysts in U.S. Pat. No. 4,720,600. The oligomerization of isobutene over an x-ray-amorphous aluminum silicate is disclosed in EP 0 536 839. Even at the mild temperatures of 60–65° C., it fails to avoid a loss of 2,2,4-trimethylpentenes by skeletal isomerization. Isobutene oligomerization over an x-ray-amorphous nickel aluminum silicate is described in WO 93/06926. This converts undiluted isobutene at 60° C. The product spectrum shows that the C8 selectivity is not particularly high. At an isobutene conversion of 15–20%, the C8 selectivity is 85–86%, and at a conversion of 75%, only 72%. In U.S. Pat. No. 3,531,539, isobutene which is in mixtures of 1-butene is converted over a molecular sieve. U.S. Pat. No. 3,518,323 discloses the conversion of isobutene which is in mixtures with n-butenes over a supported nickel catalyst. The selective dimerization of isobutene from mixtures of C4-monoolefins over heterogeneous nickel catalysts is also described by U.S. Pat. No. 3,832,418. In U.S. Pat. No. 4,197,185, the conversion of isobutene from mixtures of C4-hydrocarbons over inorganic heterogeneous catalysts is part of the claimed process.
The conversion of isobutene over acidic ion exchange resins has been known for some time and has also been used in industrial scale processes (Hydrocarbon processing, April 1973, page 171; Erdöl, Kohle, Erdgas, Petrochem. 1974, 27, Volume 5, page 240). The discussion on limiting the use of MTBE (methyl tert-butyl ether, obtained industrially from isobutene and methanol) as a fuel additive has led to processes for dimerizing isobutene again finding increased attention. For this reason, an up-to-date review of process variants for converting isobutene over acidic ion exchange resins has been published (Hydrocarbon Processing, February 2002, page 81). Depending on the isobutene content of the raw material used, different process variants are used, both in order to manage the exothermicity of the reaction, and in order to control the selectivity. The isobutene concentration is reduced mainly by adding moderators and/or diluting.
The use of moderators which are added to the reaction in order to control activity and selectivity of the catalyst is the subject-matter of various patents. Typical moderators are, for example, methyl tert-butyl ether, tert-butanol or water. The disadvantage in principle of moderators is that the moderator or subsequent products formed from it have to be removed from the product. U.S. Pat. No. 4,100,220 discloses the use of water or TBA (tert-butyl alcohol) as the moderator, while WO 01/51435 describes the use of TBA. U.S. Pat. Nos. 4,375,576 and 4,447,668 use MTBE as a moderator, GB 2325237 uses ethers and primary alcohols, EP 1074534 uses tertiary alcohols, primary alcohols and ethers, and EP 1074534 uses “oxygenates” in general. Since t-butyl ethers such as MTBE are formed from isobutene and alcohols in the presence of acidic catalysts, there also exist processes for parallel production of ethers and isobutene oligomers (U.S. Pat. No. 5,723,687).
The use of moderators additionally brings disadvantages in industrial operation. Catalysts used as ion exchangers whose activity is reduced by adding moderators such as water or TBA react very slowly on loading changes in the reactor. After the amount of feed is changed, the reactor requires time in order to return to a steady state. Steady state behavior of the reactor is advantageous for simple and safe operation of the plant. One advantage of the use of partly neutralized ion exchangers is that the time required to achieve steady state reactor behavior after a loading change is shortened.
In addition to or parallel to the use of moderators, the dilution of the isobutenic feedstock is described, in order to achieve better selectivities, but in particular control of the exothermicity (U.S. Pat. No. 5,877,372, WO 01/51435, US 2002/0002316). U.S. Pat. No. 5,003,124 describes a process in which the problem of removing the heat of reaction is countered by working only partly in the liquid phase at the boiling temperature of the liquid phase.
The documents U.S. Pat. No. 6,274,783 and WO 01/81278 both relate to processes for oligomerization with simultaneous hydrogenation of the products.
In EP 0 417 407, shaped bodies of strongly acidic ion exchangers are used as the catalyst for the oligomerization of isobutene. Some of the ion exchangers are subsequently treated with acid, in order to achieve increased acidity. The dimers yield of 93–96% is good. However, the composition of the C8 fraction is not disclosed.
EP 0 048 893 describes a process for preparing C4-oligomers and alkyl tert-butyl ethers. Isobutene is converted together with alcohols in the liquid phase over an acidic cation exchanger whose acidic sites are occupied by one or more metals of groups 7 or 8 of the Periodic Table. The metals are in elemental form.
EP 0 325 144 discloses the use of acidic ion exchangers which are partly laden with amphoteric elements (preference is given to aluminum, chromium, vanadium, titanium, zirconium, molybdenum, tungsten). These modified ion exchangers are used as catalysts in the preparation of tert-amyl alcohol (TAA) from i-amylenes. The advantages of the process are an increased conversion of the i-amylenes and simultaneous suppression of the oligomerization of the i-amylenes.
There is therefore a need for the efficient preparation of diisobutene, and the removal of isobutene to prepare 1-butene from C4-hydrocarbon mixtures.